CHOEUNG EK , Cambodia -LRB- CNN -RRB- -- More than 30 years after the genocide , the killing fields still cover Cambodia .

Him Huy : `` As soon as I got there , I felt that something was wrong . ''

Across the country , 20,000 mass graves stand as silent witnesses to the horrors that took the lives of more than 2 million Cambodians . During the rule of the Khmer Rouge , Cambodia 's lush countryside was transformed by slave labor , starvation , fear and execution .

Him Huy saw it all firsthand -- as a member of the Khmer Rouge .

In 1976 , he was part of a military group , he said , and was ultimately assigned to work at the notorious prison at Tuol Sleng .

`` As soon as I got there , I felt that something was wrong , '' he said . `` This place kills generations . We all had a sense of what was going on . I knew this place would be full of death . ''

Once a high school in the middle of Cambodia 's bustling capital city , Tuol Sleng was transformed by the Khmer Rouge into a prison and torture center .

In the hallways and classrooms , claustrophobic , rough-hewn cells and interrogation rooms were built to keep prisoners . Other prisoners were held for days and weeks at a time , packed in larger rooms and forced to lay motionless on the floor side by side .

At Tuol Sleng , so-called `` political prisoners '' were interrogated , beaten , tortured and forced to confess . The barbaric rules for prisoners were posted on the wall , and instructed them , among other things , that `` while getting lashes or electrification , you must not cry at all . '' Watch the rules for prisoners at Tuol Sleng ''

During the reign of the Khmer Rouge , almost 15,000 men , women and children were brought to Tuol Sleng . Only seven survived . The rest were executed and buried at the killing fields at Choeung Ek .

Him Huy says he was in charge of receiving prisoners , both at Tuol Sleng and when they arrived at Choeung Ek . Some workers transported prisoners to the killing fields . Then , Huy says , they `` opened the car door and gathered some cloth to tie the prisoners . Then they were led one by one into -LSB- a -RSB- house . After all the prisoners were led in , a group of us helped to guard and lead the prisoners to the ditches , where another group were waiting to kill the prisoners . ''

Him Huy says he was occasionally forced to hit prisoners at the graveside before others came to slit their throats and finish the job . Watch Him Huy demonstrate what he did at Choeung Ek ''

Some witnesses accuse Him Huy of taking an active role in the horrors at Tuol Sleng , including executing prisoners and separating infants and children from their mothers and sending them to slaughter . Him Huy said he was only doing what he was forced to do .

`` We did not do this on our own . The Khmer Rouge brought us here , and we knew nothing . And once you got here you can not leave . The tasks were assigned and told and done . So if someone was assigned to spy , they spied ; if they were assigned to guard , they guarded . ''

Him Huy said no one dared resist . `` During that time , you do what you can to survive . If you do n't do what they say , you die . ''

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Him Huy was a member of the Khmer Rouge , assigned to work at Tuol Sleng

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The notorious prison and torture center was once a Phnom Penh high school

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Some witnesses accuse Him Huy of taking an active role in the horrors there

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Him Huy says he was only doing what he was forced to do